The goal of this project is to investigate the mechanism by which v-myb transformed leukemic cells lose their neoplastic phenotype and differentiate. Retinoic acid (RA) and TPA induce the differentiation and determine the differentiation pathway of human promyelocytic cells HL-60. Avian myeloblastosis virus (AMV) transducing the oncogene v-myb induces acute myeloid leukemia. Unlike the HL-60, the v-myb transformed monoblast cell line BM2 is only TPA- but not RA-inducible to differentiate. The failure of RA-induced differentiation of BM2 cells might be due to insufficient RA receptor expression in these cells. The phenotypic changes of both human promyelocytic and murine myelomonocytic leukemia cells in response to TPA are preceded by an increase of c-jun and c-fos transcription suggesting that c-fos and/or c-jun may be required for the expression of macrophage-specific functions. Inducible increase of fos and jun expression in BM2 monocytes would demonstrate whether these two proteins are sufficient to induce the differentiation of v-myb transformed monoblasts. The opposite approach to investigate the role of jun and fos in myeloid differentiation is to express their "dominant negative" mutants in BM2 cells, thus eliminating their endogenous counterparts and potentially blocking TPA-induced differentiation. In addition, the expression of dominant negative mutant of RAR, RXR, jun and fos in primary hematopoietic cells might transform them. Therefore the studies outlined in this proposal will accomplish the following: (1) The construction of inducible forms of RAR, RXR, jun and fos and their dominant negative mutants and their stable integration into BM2 cells. (2) The TPA and RA treatment of cell lines made in (1) to test the potential changes in their differentiation pattern. (3) The preparation and screening of differential cDNA libraries to search for RAR, RXR, jun and fos target genes. (4) The preparation of viruses with dominant negative forms of RAR, RXR, jun and fos to test their potential transformation effects upon hematopoietic progenitor cells.